LONDON—United Co. Rusal said Friday it is seeking a British court order to push through a $3.6 billion debt refinancing deal, after the aluminum giant failed to win the backing of some Western creditors.

Although the vast majority of Rusal's mostly Russian lenders have agreed to the new arrangement, two Western European creditors—Royal Bank of Scotland, now majority-owned by the U.K. government, and Erste Abwicklungsanstalt, the...